Internal Combustion Engines (ICE's) are used for generating power in a variety of applications, such as vehicles and engine generators for generating electricity. For example, ICE's are used for generating power in vehicles such as automobiles, trucks, trains, armored vehicles, tractors, boats, submarines, and aircraft. Research is continually being conducted to improve ICE efficiency. ICE researchers, designers and manufacturers seek to balance efficiency gains with resulting emissions, as more stringent emissions standards are being implemented throughout the world.
In general, there are two broad types of ICE's that differ by the combustion initiating source, for example, spark-ignition or compression ignition. Spark-ignited engines rely on a high energy discharge to ignite the fuel with the air present in the combustion chamber of the engine. The fuel used with spark-ignited engines typically has ignition characteristics that require high temperatures to initiate the ignition. For example, spark-ignited engines often use fuels such as gasoline, natural gas and/or ethanol, which all have poor autoignition characteristics (e.g., relatively higher octane numbers than fuels used for compression ignition).
Alternatively, a compression ignition engine (e.g., a diesel engine) is an internal combustion engine that uses the heat of compression to initiate ignition to burn fuel that has been injected into the combustion chamber. This is in contrast to spark-ignition engines which use a spark plug to provide a high energy discharge to ignite an air-fuel mixture. Compression ignition engines, rely on the fuel to ignite within the operating conditions of the engine. There are a number of fuels that are suitable for compression ignition engines such as, but not limited to, diesel fuel and some modern “biofuels.”
The two main types of ICE's (e.g., spark-ignition and compression ignition) differ in other characteristics as well. For example, spark-ignition engines and compression ignition engines typically operate using different compression ratios. One feature of compression ignition engines is that they have better optimized engine compression ratios (e.g., higher compression ratios) that lead to higher engine efficiencies. Spark-ignited engines on the other hand, tend to have lower compression ratios in an effort to limit autoignition of the fuel during operation. Compression ignition engines are typically more efficient than spark-ignited engines as a result of the higher compression ratios employed by compression ignition engines.
Additionally, emissions concerns vary for both types of ICE's. Compression ignition engines using diesel fuel tend to have greater concerns with the emission of particulate matter and nitrous oxides (due to the high combustion temperatures often obtained). Spark-ignited engines using, for example, gasoline fuel, often have concerns regarding the emission of unburnt hydrocarbon compounds, which may include a variety of hydrocarbon compounds, and carbon monoxide. As such, the concerns regarding emissions are often linked to the fuel being used. Consequently, understanding how engines obtain enhanced efficiencies and lower emissions can lead to improvements in engine operation obtained by fuel optimization.
Currently, dual- or bi-fuel systems are being used with compression ignition engines to enable clean and efficient combustion. The particular fuel composition of the dual- or bi-fuel system may depend upon the operating condition of the engine with which it is being used. For example, under high load conditions engines have high in-cylinder temperatures and compression ignition is easier to achieve. Thus, under high load conditions, the dual- or bi-fuel composition may be formulated to be less susceptible to compression ignition (e.g., it may be formulated to be compressed further than other formulations of the fuel of the dual- or bi-fuel before ignition will occur) may be used.
In contrast, in-cylinder temperatures are much lower under low load conditions and, therefore, compression ignition is more difficult to achieve. Although in-cylinder temperatures remain low under low load conditions, for compression ignition engines the fuel still needs to be able to ignite as a result of compression. As such, a dual-fuel system typically uses one fuel to allow compression ignition under low, hard to ignite, engine load conditions, and a different fuel under the high, relatively easier to ignite, engine load conditions.
An example of a dual-fuel system being used with a compression ignition engine may currently be found in the area of generators. For example, some dual-fuel generators use a combination of diesel fuel and natural gas. Due to its relatively lower price and cleaner combustion characteristics, natural gas is used as the bulk fuel component, while the diesel fuel is used primarily as a ‘pilot’ or ignition source when compression ignition is more difficult to achieve. These types of systems are costly and not preferred due to the need for two fuel sources.